Design columns

Height of column = 30 m Working load = 1.4 x 4.8 For grade 50 steel 

Design strength Py = 265 N/mm Effective length Le = Q.IL = 0.7 X 30 = 21 m Design load = 1814.4 Column strength = PcAg 

Actual pressure under plate = 500x 5 0 7.26 N/mm Plate thickness 

Minimum value of elastic modulus Z = 365 cm Moment capacity of major axis McX = 363 X = 163.35KNm The applied axial load /= (2 X 120 + 590) X 0.0088 = 7.30 KNm Substitute 

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The FUG method is convenient for new-column design with the following specifications ... 

FIG. 19-79 Variations in the basic column design a) packed column, (h) Jameson cell, and (c) air-sparged hydrocyclone. 

A more quantitative and lengthy method, but still very useful for checking of the type required here is the Smith-Brinkley method (Reference 5). It uses two sets of separation factors for the top and bottom parts of the column for a fractionator or reboiled absorber and one overall separation factor for a simple absorber. The method is tailor-made for analysis of a column design or a field installed column. The Smith-Brinkley method starts with the column parameters and calculates the resulting product compositions unlike other methods that require knowing the compositions to determine the required reflux. 

There are two fundamental chromatography theories that deal with solute retention and solute dispersion and these are the Plate Theory and the Rate Theory, respectively. It is essential to be familiar with both these theories in order to understand the chromatographic process, the function of the column, and column design. The first effective theory to be developed was the plate theory, which revealed those factors that controlled chromatographic retention and allowed the... 

It appears that the equation introduced by Van Deemter is still the simplest and the most reliable for use in general column design. Nevertheless, all the equations helped to further understand the processes that occur in the column. In particular, in addition to describing dispersion, the Kennedy and Knox equation can also be employed to assess the efficiency of the packing procedure used in the preparation of a chromatography column. 

The composite envelope is then plotted over the envelope of each individual peak. It is seen that the actual retention difference, if taken from the maxima of the envelope, will give a value of less than 80% of the true retention difference. Furthermore as the peaks become closer this error increases rapidly. Unfortunately, this type of error is not normally taken into account by most data processing software. It follows that, if such data was used for solute identification, or column design, the results can be grossly in error. 

Equation (16) was first developed by Purnell [3] in 1959 and is extremely important. It can be used to calculate the efficiency required to separate a given pair of solutes from the capacity factor of the first eluted peak and their separation ratio. It is particularly important in the theory and practice of column design. In the particular derivation given here, the resolution is referenced to (Ra) the capacity ratio of the first... 

Giddings made a stalwart effort to provide values for the different constants that would apply to diverse stationary phase and support conditions [2]. However, at best, his values are the closest estimates from an assumed set of conditions that may fit, to a greater or lesser extent, the properties of the actual stationary phase or support in use. In some cases, his constants may be used in column design and to help in the choice of those operating conditions that will provide the required... 

It is important to appreciate that, in all aspects of column evaluation and column design in GC, the compressibility of the mobile phase must be taken into account or serious enors will be incurred. Either equation (13) or (15) can be employed but, as already stated, equation (13) is recommended as the more simple to use. 

Atwood and Goldstein [16] examined the effect of pressure on solute diffusivity and an example of some of their results is shown in Figure 7. It is seen that the diffusivity of the solutes appears to fall linearly with inlet pressure up to 40 MPa and the slopes of all the curves appear to be closely similar. This might mean that, in column design, diffusivities measured or calculated at atmospheric pressure might be used after they have been appropriately corrected for pressure using correction factors obtained from results such as those reported by Atwood and Goldstein [16]. It is also seen that the... 

It would appear, from the data available at this time, that the Van Deemter equation would be the most appropriate to use in column design. 

Finally, the speed of response of the detector sensor and the associated electronics once played an important part in optimum column design. The speed of response, or the overall time constant of the detector and associated electronics, would be particularly important in the analysis of simple mixtures where the analysis time can be extremely short and the elution of each peak extremely rapid. Fortunately, modern LC detector sensors have a very fast response and the associated electronic circuits very small time constants and, thus, the overall time constant of the detector system does not significantly influence column design in contemporary instruments. The instrument constraints are summarized in Table 2... 

Whether the optimum phase system is arrived at by a computer system, or by trial and error experiments (which are often carried out, even after computer optimization), the basic chromatographic data needed in column design will be... 

Column design involves the application of a number of specific equations (most of which have been previously derived and/or discussed) to determine the column parameters and operating conditions that will provide the analytical specifications necessary to achieve a specific separation. The characteristics of the separation will be defined by the reduced chromatogram of the particular sample of interest. First, it is necessary to calculate the efficiency required to separate the critical pair of the reduced chromatogram of the sample. This requires a knowledge of the capacity ratio of the first eluted peak of the critical pair and their separation ratio. Employing the Purnell equation (chapter 6, equation (16)). 

Equation (13) is the first important equation for open tubular column design. It is seen that the optimum radius, with which the column will operate at the optimum velocity for the given inlet pressure, increases rapidly as an inverse function of the separation ratio (cc-1) and inversely as the square root of the inlet pressure. Again it must be remembered that, when calculating (ropt)5 the dimensions of the applied pressure (P) must be appropriate for the dimensions in which the viscosity (r)) is measured. 

In a packed column the HETP depends on the particle diameter and is not related to the column radius. As a result, an expression for the optimum particle diameter is independently derived, and then the column radius determined from the extracolumn dispersion. This is not true for the open tubular column, as the HETP is determined by the column radius. It follows that a converse procedure must be employed. Firstly the optimum column radius is determined and then the maximum extra-column dispersion that the column can tolerate calculated. Thus, with open tubular columns, the chromatographic system, in particular the detector dispersion and the maximum sample volume, is dictated by the column design which, in turn, is governed by the nature of the separation. 

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